HW3
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Recursion
(1) Ex  2 (paper)
(2) Ex (Maude): Define PCF using any of the semantic styles discussed
    at the begining of this class (smal-step, big-step, etc.)
(3) Ex 10 (paper)
(4) Ex 14 (paper)

Polymorphism
(5) Ex  2 + 14 (Maude); test them using the reverse in Ex 10
(6) Prop 6 (paper)
(7) Ex 20 + 21 (Maude)
